1. Field of the Invention
The present invention relates to a shield tunneling method for constructing a small diameter tunnel with an inside diameter which is typically on the order of 1.5 m or less, by assembling a set of three 120.degree. arcuate segments into a so-called segment ring. The present invention also pertains to a shield machine for carrying out the method.
2. Statement of the Related Art
For constructing a small diameter tunnel with an inside diameter on the order of 1.5 m or less, it is known to use a shield machine for either manually or mechanically excavating the ground face in front of the shield machine, substantially under unpressurized condition, to assemble within the shield machine a set of three 120.degree. arcuate segments into a so-called segment ring, and to connect back-filling pipes with grout holes extending through the wall of the segments for charging a back-filling material, e.g. rounded gravels, grout or the like, into a so-called tail void which is an annular gap defined between the outer periphery of the segment ring and the surrounding soil. In this connection, reference may be had to U.K. Patent Specification Nos. 1,276,618; 1,288,393 and 1,288,394.
In the case of a shield tunneling method using the 120.degree. arcuate segments as mentioned above, in order to allow an assembly within the shield machine of a set of segments into a segment ring, it is necessary for the radial dimension of the tail void to be at least about one half of the thickness of the segment, and to be typically as large as the segment thickness. Then, unlike in a conventional method wherein a segment ring is formed of four or more segments, it is of particular importance to uniformly charge the back-filling material into the tail void, and to achieve a sufficient hardening of the charged material therein.
When the shield tunneling method is carried out in a poor ground, it is generally necessary to remove the underground water around the face in order to prevent collapse of the face. To this end, it is a conventional practice to take various auxiliary measures together, such as chemical grouting method, pneumatic method, dewatering method or the like, either of which calls for complicated works thereby leading to a prolonged working period and a resultant increase in the working cost.
In a conventional shield tunneling method, particularly when the work is to be conducted in the poor ground, an earth pressure balancing method is sometimes used together wherein mud making agents are supplied to the face to balance the resulting force with the pressure applied to the face. The earth pressure balancing method is often advantageous in that, as compared with the above-mentioned auxiliary measures, it is far economical and yet allows the required work to be performed in a facilitated manner. However, it has been generally considered difficult to apply the earth pressure balancing method in combination with the small-diameter shield tunneling method using the 120.degree. arcuate segments. This difficulty is closely related to the unique arrangement inherent to the small-diameter shield tunneling method wherein the tail void has a substantial radial dimension. That is, after the back-filling material has been charged into the tail void, the back-filling material often exhibits a problematic tendency to collapse prior to completion of its hardening, which is mainly due to a substantial amount of mud making agent with a high water content, and which more or less results in that the required grouting function of the back-filling material cannot be sufficiently achieved.